During the combustion of fossil fuels, including gaseous fuels such as natural gas, liquefied natural gas and propane, for example, in air, NO.sub.x is formed and emitted to the atmosphere in the combustion products. With respect to gaseous fuels that contain little or no fuel-bound nitrogen per se, NO.sub.x is formed as a consequence of oxygen and nitrogen in the air reacting at the high temperatures resulting from the combustion of the fuel.
Governmental agencies have passed legislation regulating the amount of oxides of nitrogen that may be admitted to the atmosphere during the operation of various combustion devices. For example, in certain areas of the United States, regulations limit the permissible emission of NO.sub.x from residential furnaces and water heaters to 40 ng/J (nanograms/Joule) of useful heat generated by these combustion devices. It is expected that future regulations will restrict NO.sub.x emissions from residential furnaces, water heaters and boilers to even lower levels.
Gas fired apparatus, such as residential and light commercial heating furnaces, often use a particular type of gas burner commonly referred to as an in-shot burner. An in-shot burner comprises a burner nozzle having an inlet at one end for receiving separate fuel and primary air streams and an outlet at the other end through which mixed fuel and primary air discharges from the burner nozzle in a generally downstream direction. The burner nozzle may comprise simply comprise an axially elongated, straight tube, or it may comprise a generally tubular member, which may be arcuate or straight, having an inlet section, an outlet section and a transition section, commonly a venturi section, disposed therebetween. Fuel gas under pressure passes through a central port disposed at or somewhat upstream of a fuel inlet to the inlet of the burner nozzle. The diameter of the inlet to the burner nozzle is larger than the diameter of the fuel inlet so as to form an annular area through which atmospheric air is drawn into the burner nozzle about the incoming fuel gas. This primary air mixes with the fuel gas as it passes through the tubular section of the burner nozzle to form a primary air/gas mix. This primary air/gas mix discharges from the burner nozzle through the outlet of the burner nozzle and ignites as it exits the nozzle outlet section forming a flame projecting downstream from a flame front located adjacent or somewhat downstream of the outlet of the burner nozzle. Secondary air flows around the outside of the burner nozzle and is entrained in the burning mixture downstream of the nozzle in order to provide additional air to support combustion.
In conventional practice, a flame retention device is often inserted within the outlet section of the burner in an attempt to achieve improved flame stability and reduction of noise. One known insert comprises a cylindrical body defining a central opening and having a toothed perimeter formed by a plurality of circumferentially spaced, axially elongated splines extending radially outwardly in a sunburst pattern about the circumference of the cylindrical body. U.S. Pat. No. 5,108,284, Gruswitz, for example, discloses an in-shot burner having a sunburst type flame retention device wherein each spline comprises an axially elongated bar of rectangular cross-section. U.S. Pat. No. 5,791,893, Charles, Sr. et al., discloses an in-shot burner having a porous silicon carbide ceramic flame retention insert located in the outlet section of the burner nozzle. Another known insert has a central opening surrounded by a series of circumferentially spaced, small holes.
U.S. Pat. No. 4,776,320, Ripka et al., discloses a gas-fired furnace utilizing an in-shot burner wherein a thermal energy radiator structure, such as a perforated stainless steel tube, is disposed in the flame downstream of the burner outlet. The radiator structure tempers the flame by absorbing heat therefrom and radiating the absorbed heat to the surrounding heat transfer surface, whereby peak flame temperatures are limited and residence times at peak flame temperature are reduced.
U.S. Pat. No. 5,333,597, Kirkpatrick et al., discloses a gas-fired furnace utilizing an in-shot burner wherein a porous NO.sub.x abatement member is disposed in the flame downstream of the burner outlet. The combustion flame and combustion products pass through the porous NO.sub.x abatement member, whereby peak combustion temperatures and residence times at peak temperatures are reduced. The preferred NO.sub.x abatement member is stated to be a metallic screen since metals are good thermal conductors and radiators, although ceramic refractory materials are also stated to be acceptable.
U.S. Pat. No. 5,370,529, Lu et al., discloses a gas-fired furnace wherein a mesh tube is disposed in the flame downstream of the burner outlet. During operation of the burner, the flame passes through the mesh tube, thus reducing the cross-section of the flame, increasing the axial velocity of the flame, and substantially diminishing contact of the secondary combustion air with the maximum temperature zones of the flame, whereby NO.sub.x formation is said to be inhibited.
U.S. Pat. No. 5,244,381, Cahlik discloses a gas-fired furnace utilizing an in-shot burner wherein a flame spreader, which in the depicted embodiment comprises a stainless steel plate having a plurality of stainless steel rods mounted on its face, is disposed in the flame downstream of the burner outlet. The flame spreader is said to absorb flame heat energy and lower the temperature of the flame, so as to reduce NO.sub.x formation in the flame.
A problem associated with the reduction of nitrogen oxide formation by lowering the flame temperature is that as the flame is quenched, combustion efficiency is reduced and combustion may not be totally completed. As a consequence of flame quenching, carbon monoxide formation will increase as nitrogen oxide formation decreases.
U.S. Pat. No. 5,174,744, Singh, discloses an industrial gas-fired burner wherein a block of highly porous reticulated ceramic foam is disposed in spaced relationship to and downstream of the burner nozzle. The burner is operated so as to produce a low temperature flame resulting in lower NO.sub.x emissions but also increased carbon monoxide emissions. The incompletely combusted carbon monoxide passes through the ceramic foam block and is said to be oxidized into carbon dioxide by oxygen in the surrounding air as it traverses the hot foam block.
U.S. Pat. No. 5,848,887, Zabielski et al., discloses a low emission combustion system for a residential heating furnace including both a radiator body and a catalyst. The radiator body is disposed in the flame downstream of an in-shot burner to quench the flame to reduce NO.sub.x formation, while the catalyst is disposed further downstream of the flame in a lower temperature region for oxidizing carbon monoxide in the flue gas to carbon dioxide.
To avoid the consequence of increased carbon monoxide formation associated with reduction of NO.sub.x emissions by reducing peak flame temperatures, attempts have been made to reduce nitrogen oxides formation by using a catalyst to promote chemical reactions which result in a reduction of NO.sub.x formation in the flame. U.S. Pat. No. 5,746,194, Legutko, discloses a combustion system having an in-shot burner wherein a flow dividing member supports a partial oxidation catalyst disposed in the fuel rich inner core of the flame downstream of the burner outlet. The catalyst serves to catalyze unburnt methane in the fuel rich inner core of the flame to hydrogen and carbon monoxide. When this hydrogen and carbon monoxide subsequently combust in the air rich outer zone of the flame, the peak combustion temperatures are lower than in conventional combustion and NO.sub.x formation is reduced. The catalytic insert is heated above the reaction "light-off" temperature of the catalyst directly by the flame itself. The catalytic insert also radiates heat away from the flame to further reduce peak temperature within the flame.